FI115683B - Synchronization procedure and information transfer system - Google Patents

Description

115683

Synchronization method and communication system

Area

The present invention relates to a communication system and a synchronization method in a communication system using multicast transmission.

5 Background

Wireless communication systems have recently been the subject of rapid development. Numerous new services have been developed, and there has been a need to develop solutions that enable high data throughput. In addition to commonly used FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access) methods of multi-carrier transmission have been developed. In these methods, the signal is transmitted simultaneously using several different carriers.

Multicarrier signals are used especially in wide bandwidth donation over a fading multi-path radio channel. Multipath propagation means that the signal travels between the transmitter and the receiver through several different paths. Splitting the signal into multiple parallel carriers extends the duration of symbols transmitted in the radio channel and enables frequency domain coding, whereby interference caused by multipath propagation can be eliminated without substantially reducing data transmission capacity.

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In a digital multicarrier system, the information is distributed to a plurality of parallel carriers in the frequency domain, followed by a conversion to a time domain, followed by a signal being transmitted. Timing, or synchronous ';'; 'Ringing is essential for data transmission, and especially for multicarrier transmissions. hours

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In prior art solutions for synchronization, a few known symbols are created which are transmitted in the frequency domain modulated before the actual data. The receiver can recognize the synchronization symbol and · ,; · Start the reception on time. Timing accuracy is important because the receiver's time domain signal is again converted to frequency. '30 soon for expression.

,,; In prior art solutions, synchronization typically occurs using different correlation methods. Known methods include, for example, the use of a differential correlator and the use of a fitted filter.

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In differential correlation, the signal is divided into two parts, one of which is delayed and correlated with the non-delayed part. If the signal received and sampled by the receiver consists of two consecutive identical sequences, the output of the correlator will result in a peak 5 exceeding the predetermined threshold level, the transmission is expected to arrive and reception is initiated.

When using a matched filter, the correlation between the incoming frequency modulated signal and the coefficients pre-programmed into the filter is calculated. When the signals are sufficiently similar, the output of the 10 matched filters produces a peak which is compared to a threshold level above which reception is initiated.

The disadvantages of prior art methods include poor timing accuracy, implementation complexity, slow synchronization, or reliability.

In addition to time synchronization, there is also the problem of frequency synchronization, since the orthogonality of the multi-carrier signal is significantly reduced due to a frequency error, thus impairing performance. In addition, the function of the phase-accurate signal, or coherent detection, of the signal requires that the transmitter and receiver operate very accurately at the same frequency. In practice this is not possible, but generally the 20 receivers have to measure the frequency difference between the transmitter and the receiver from the received signal and correct their own frequency or incoming sig - ..! 'To compensate for the frequency difference.

Adaptive systems that vary, for example, the modulation scheme used for signal transmission or other transmission parameters, have the drawback that the transmitted signal component must be allocated a · ··· transmission of information needed for adaptation. which reduces the bandwidth efficiency.

BRIEF DESCRIPTION: It is an object of the invention to provide an efficient synchronization solution.

This is achieved by a synchronization method in a data transmission system in which burst data is transmitted in the frequency domain modulated on a plurality of carriers. At the transmitter, a pseudorandom bit string transmitted at one carrier before the other data is appended to the beginning of each burst, and,. After the pseudorandom bit string, at least one frequency-modulated sync pattern transmitted by multiple carriers is used by the receiver. and frequency synchronizing with the received signal.

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The invention also relates to a communication system comprising a transmitter and a receiver, which transmitter is adapted to transmit burst-like data in the frequency domain modulated on a plurality of carriers. The transmitter comprises means for coupling at the beginning of each burst a time domain modulated pseudorandom female bit string, and the transmitter is adapted to transmit a bit string before the data using a single carrier, and the transmitter is adapted to append at least one frequency modulated synchronization pattern after the bit string; transmit a synchronization pattern using multiple carriers, and the receiver comprises means for using a pseudo-random bit-10 sequence in time and frequency synchronization with the received signal.

The invention also relates to a transmitter adapted to transmit burst data in the frequency domain modulated to a plurality of carriers. The transmitter comprises means for appending at the beginning of each burst a time-modulated random bit sequence and, after the pseudorandom bit sequence, at least one back-15 modulated synchronization pattern, and the transmitter is adapted to transmit the bit sequence before other data using a single carrier.

The invention also relates to a receiver adapted to receive a burst-like signal transmitted in the frequency domain modulated on a plurality of carriers. The receiver comprises means for detecting from the beginning of each burst a bit carrier transmitted on it, and a synchronization pattern transmitted on multiple carriers, and means for using a pseudorandom bit sequence in time and frequency synchronization with the received signal.

Preferred embodiments of the invention are described in the dependent claims. 25.

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The solution according to the invention achieves several advantages. The pseudorandom PN sequence may be implemented at a time domain shorter than the frequency modulated synchronization symbol, whereby synchronization is faster than previously known methods. Also: the accuracy of the synchronization is better than the accuracy of the synchronization by means of, for example, a differential correlator. When using binary bit strings, the implementation of the matched filter used to receive is easy and costly. low computing power compared to the case where the frequency modulated] · 'signal is correlated as a matched filter.

35 The peak power ratio of the multicarrier signal to the average power is very high, i.e., there are strong transient peaks in the transmission, even if the average signal level of 115683 4 is low. The linearity of the transmitter and receiver components must be dimensioned to meet peak power requirements. The envelope of the pseudorandom PN signal is almost constant, i.e. the ratio of peak power to average power is small. In this case, the PN sequence can be transmitted at a significantly higher average power than the actual data, thereby making the synchronization very reliable.

List of figures

The invention will now be described in greater detail in connection with preferred embodiments, with reference to the accompanying drawings, in which Figs. the third example of a receiver.

Description of Embodiments

Advantageous embodiments of the invention can be applied to multicast transmission systems in which burst-like data is transmitted. Referring to Figure 1A, an example of burst construction and multi-carrier transmission is considered. Fig. 1A shows a burst 100. The burst comprises the actual data to be transmitted 102, which consists for example of one or more consecutive symbols. Prior to the data, the burst comprised at least one synchronization symbol 104 used by the receiver for channel estimation. The synchronization symbol may be, for example, a symbol whose phase and amplitude are known in advance. In the preferred embodiments, a pseudorandom bit string 106 is attached to the beginning of the burst before the synchronization symbols, which is, for example, a binary bit string.

Thus, in a multi-carrier system, the signal to be transmitted is distributed to a plurality of parallel carriers in the frequency domain, followed by a con-30 version in the time domain, after which the signal is transmitted. Fig. 1A illustrates transmission carriers 108-118 for transmission. The horizontal axis has I · **; · time and the vertical axis has frequency. The number of carriers and the frequency difference between them are system dependent parameters such as those skilled in the art. *: it is clear.

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In burst 100 transmission, bit string 106 at the beginning of burst is modulated, for example, by the BPSK method and transmitted using a single carrier 112. Thereafter, the remainder of the burst, i.e. one or more synchronization symbols 104 and the actual data, are dedicated to the dedicated carriers 108-118 and transmitted to the receiver. The bit sequence 106 may consist of one or more BPSK encoded binary pseudorandom sequences.

Figure 1B illustrates another embodiment in which a bit sequence 106 of a burst 100 is transmitted in a so-called. 0 carrier 120 located at the center frequency of 10 transmissions. The number of frequencies 122 and 124 on both sides of the center frequency is thus equal. In one embodiment, the carrier 120 used to transmit the bit string does not transmit the actual data.

Figure 1C illustrates an example of a signal burst in a time domain. The horizontal axis of the pattern has time and the vertical axis has amplitude. The burst comprises a bit string 106, synchronization symbols 104, and actual data 102. The figure illustrates how the data signal 102 has a high peak-to-average ratio. The data signal thus comprises strong instantaneous peaks. In this case, the transmitter and receiver components must be dimensioned sufficiently linear for the maximum signal. The bit string 106, in turn, does not vary in amplitude, so it can be transmitted at a higher average power than the data signal 102. This improves the reliability of the synchronization.

Figure 2 illustrates an example of a transmitter. The transmitter comprises a modulator 200 having input 202 to be transmitted.

In the modulator 200, data is processed into blocks of a selected length (e.g., * · * .25 to 400 bits, but the length of the blocks may also have any other value). Each block may be encoded, for example, by a convolutional encoder and ..! that is, mixing the order of the bits of the block with the desired sequence. In this connection, data is distributed to different carriers. The blocks are then modulated and pre-distorted. Modulator 200 is well known to one of ordinary skill in the art: · 30 known. At the output of the modulator are modulated complex symbols: 204, which are applied in the desired order to a converter 206, where symbols v are converted from frequency levels to time domain, for example, by means of an IFFT (Inverse Fast Transform). Prior to the actual data symbols, the converter 206 is supplied with the desired number of known synchronization symbols that the receiver uses -...: 35 for signal detection.

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The complex output signal 208 of the converter is further passed to a signal processing block 210 implemented, for example, by a processor and signal processing software or programmable logic. Other alternatives are possible, as will be apparent to one skilled in the art. In signal concept block 5, block 210 may modify the complex signal as desired, for example, by compressing, cutting, filtering, or buffering.

The transmitter also comprises a control unit 212 which controls the operation of all blocks required for signal processing. The control unit also controls the operation of the bit space block generator 214. The bit block generator 214 generates, under the control of the control unit 10, a selected pseudorandom bit sequence which is appended to the signal before the actual data and frequency domain modulated synchronization symbols.

The complex mixer 216 transmits the complex signal to the desired intermediate frequency. This operation can also be performed analogously in the radio-15 frequency sections. The intermediate frequency signal is converted to analog form in the D / A converter 218 and fed to radio frequency sections 220 for amplification and mixing at the actual transmission frequency. Finally, the signal is transmitted by antenna 222.

In one embodiment, the control unit selects a bit string to be transmitted at the beginning of each burst 20 from a plurality of options. Based on the selection, the control unit transmits the corresponding control to the bit block generator 214, which generates the desired bit string. The bit string can thus be used to transfer any desired information to the receiver. For example, in adaptive systems that • vary the modulation method used in transmission, the information about the modulation method used can be transmitted to the receiver using a specific bit string.

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Figure 3 illustrates an example of a receiver. The structure to be presented is particularly suitable for embodiments in which the bit string used is not interchanged. The receiver receives the signal by antenna 300. The received signal 302 is transmitted by radio frequency sections 304, where the signal is typically amplified, filtered and converted to intermediate frequency. Intermediate frequency sig- '. the signal is output to an analog / digital converter 306 where the signal is converted to digital format. From the output of the converter, the signal is applied to mixer 308, where the signal is complexly mixed to the baseband, whereby the signal: 35 is formed by its I and Q portions having a 90 degree phase shift. This operation * can also be performed analogously in the radio frequency sections. The baseband signal 115683 7 is applied in parallel to the signal processing block 310 and the matched filter 312. The signal processing block 310 is implemented, for example, by means of a processor and signal processing software or programmable logic. Other alternatives are possible, as will be apparent to one skilled in the art.

5 A matching filter 312 synchronizes the receiver to the bit string at the beginning of the burst. The filter calculates the correlation between the pre-programmed PN sequence and the I and Q portions of the received signal. The output of the filter 312 is coupled to the control unit 314, where a squared sum power is calculated from the correlation result of the I and Q sections. The calculated sum-10 mate power is compared to a preset threshold, and when the sum power value exceeds a threshold, the control unit detects an initial burst reception.

In the signal processing block 310, the complex signal can be processed as desired, for example, a frequency error is determined from the signal. When the frequency error 15 is known, it can be used to perform equalization in the mixer 308. Thus, synchronization can be performed on the equalized signal which improves synchronization.

From the output of the control unit 314, a synchronization signal 316 is applied to the converter 318 and the demodulator 320. The received 20 in the converter 318 is converted from a time domain into a frequency domain by means of, for example, Fast Fourier Transform (FFT). The input of converter 318 has an output signal of signal processing block 310; When the converter 318 receives from the control unit 314 a synchronization signal 316 when the burst starts, the converter may immediately synchronize with the burst; ; or one or more synchronization symbols. Syncronizing- , . For example, symbols are symbols whose phase and amplitude are known in advance. The receiver uses synchronization symbols e.g. channel estimation,

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that is, measuring the signal distortion caused by the radio path. Thus, the receiver can detect and compensate for phase and amplitude changes in the signal path. After the synchronization symbols, the converter receives the actual data. From the output of the converter 318 to the frequency domain, the converted signal is applied to a detector 320 which performs channel decoding, de-interleaving and decoding of carriers according to the prior art. Each burst consisted of at least one frequency domain modulated synchronization pattern.

: 35 Figure 4 illustrates another example of a receiver.

The structure shown is particularly suited to embodiments in which the 115683 8 bit strings used are interchanged. In this example, the receiver end is similar to the previous example. The operation of the antenna 300, the radio frequency components 304, the analog / digital converter 306 and the mixer 308 is similar to that described above. The baseband signal from the output of mixer 308 is applied to signal-5 processing block 310 and a plurality of matched filters 400-406.

The output of each filter 400-406 is coupled to control unit 314. The filters are pre-programmed with parameters corresponding to the bit strings used in the transmission. When the received bit string is supplied to matched filters 400 to 406, a peak is obtained from the output of the matched filter which comprised the parameters of that bit string. In the control unit 314, the output signal of each filter is compared to a preset threshold, and when the output of a filter exceeds a threshold, the control unit detects the start of burst reception and which filter detected the bit string, the control unit can determine which bit string was involved. The received signal can now be processed in the signal processing block 310, the modifier 318 and the demodulator 320 and other blocks according to information received from the control unit.

In one embodiment, the receiver uses an array of antennas or antenna elements to receive signals. Here, the directionality of the antenna array can be adjusted by phasing the signals received by the various antenna elements and summing the signals together. The known bit string attached to the beginning of each burst in the receiver ..T can be used to determine the phase difference between signals from different antenna elements.

This information can be utilized by the control unit to determine the appropriate::: steps to obtain the desired orientation pattern.

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Figure 5 illustrates an embodiment using an antenna array.

i '. *: The signal is received by the antenna array, ·· *, which consists of several antenna elements. The signal received by the antenna array 500 is applied through radio frequency sections 502 and A / D converter 504 to mixer 506. The operation of these sections is similar to that described above. The receiver also comprises an adder 508, 30 in which the signals received by different antenna elements are phased and summed based on control from the control unit 314. The matched filter 312 determines the original phase of the received signal for each antenna element, and this information is transmitted to the control unit 314, which utilizes it to determine the phase required to obtain the desired directional pattern. The signal from the adder is further forwarded to the signal processing block 310.

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Although the invention has been described above with reference to the example of the accompanying drawings, it is clear that the invention is not limited thereto, but can be modified in many ways within the scope of the appended claims.

Claims (16)

1. Synchronization method in an information transmission system, where data in burst form is transmitted in the frequency plane modulated to several carriers, characterized by the transmitter. 102), and after the pseudobit sequence (106), at least one synchronization pattern (104) modulated in the frequency plane is transmitted using multiple carriers, and the receiver uses the bit sequence when it is synchronized with the received signal in time and frequency. Method according to claim 1, characterized in that the bit sequence (106) consists of one or more BPSK-encoded binary pseudo sequences. Method according to claim 1, characterized in that the receiver uses at least one synchronization pattern (104) for channel estimation. Method according to Claim 1, characterized in that the transmitter has a plurality of bit sequences at its disposal, and with the sequence of bit sequences to be used at the beginning of each burst, information is transmitted to the receiver. 20 Method according to claim 1, characterized in that the; 1 the receiver is synchronized by means of a filter adapted in the bit sequence. •: · 6. A method according to claim 1, characterized in that shear. '. the clean frequency error is calculated in the receiver using the bit sequence. . ·· 1, 7. A method according to claim 6, characterized in that the required frequency correction is determined by means of the calculated frequency error. 8. A method according to claim 1, characterized in that avatar bursts are received in the receiver with an antenna group (500) consisting of more than one antenna element, and that in the receiver the phase of the antenna group is controlled by means of 30 bit sequences received at the beginning. in the bursts. An information transmission system, comprising a transmitter and a receiver, which transmitter is arranged to transmit burst data in frequency plan modulated to several carriers, characterized in that the transmitter comprises: means (214) for attaching to the beginning of each burst a modulated pseudo-; And that the transmitter is arranged to attach to the beginning of each burst after the bit sequence at least one synchronization pattern modulated in the frequency plane (106), and that the transmitter is arranged to transmit the synchronization pattern using multiple carriers, and the receiver comprises means (312, 314) for using the random bit sequence when synchronized with the received signal in time and frequency. 10. A system according to claim 9, characterized in that the receiver is arranged to use at least one synchronization pattern for channel estimation. System according to claim 9, characterized in that the transmitter comprises means (212, 214) for selecting the bit sequence to be transmitted at the beginning of each burst among several alternatives. System according to claim 9, characterized in that the receiver is arranged to identify the transmitted bit sequence by means of the adapted filter (312). System according to claim 11, characterized in that the receiver comprises an adapted filter (400 - 406) for each possible bit sequence to be transmitted. ... V 14. A system according to claim 9, characterized in that the receiver comprises an antenna group (500) and that the receiver comprises means (502): for controlling the phasing of the antenna group by means of the bit sequences received at the beginning of the bursts. . Transmitter arranged to transmit data in burst form in the frequency '· ·', the plane modulated into several carriers, characterized in that the transmitter recovers means (214) to modulate the beginning of each burst one in the schedule; : Pseudobit sequence (106), and after the pseudobit sequence, there is at least one synchronization pattern (104) modulated in the frequency plane and that the transmitter is arranged to transmit the bit sequence before other data using a carrier and the synchronization pattern using multiple carriers. 16. Receiver arranged to receive a signal in burst form, the signal of which has been transmitted modulated in the frequency plane to several carriers, known as the receiver comprising means (314, 400-406) for identifying from the beginning of each burst, a bit sequence transmitted thereto on a carrier, and a synchronization pattern transmitted on several carriers, and means (312, 314) for using the random bit sequence when it is synchronized with the received signal in time and frequency.